But consider his body measurements a little closer and it becomes clearer why Phelps is dominating these Olympic Games.

He has an extended trunk and relatively short legs, a distinct advantage in the water. The inseam of his pants is reportedly 32 inches (81 centimeters), shorter than that of Hicham El Guerrouj, the great Moroccan runner, who is 5 feet 9 inches (175 centimeters) but all legs.

Phelps has double-jointed elbows, knees, and ankles, which allows him to bend himself like few swimmers can. His size-14 (European-size 48.5) feet are like giant fins.

Add to that the extraordinary work rate of his lungs and heart and Phelps appears almost superhumana different species from the rest of us.

Of course, he also trains extraordinarily hard. But so do others. To be an Olympic champion, a person's genes must first be preset for maximal athletic performance. After all, great athletes are born, then made better.

"The best athletes in the world are a result of good genes and optimal training," said Phillip B. Sparling, who is a professor of applied physiology at the Georgia Institute of Technology in Atlanta. "A person who has great dedication, motivation, and excellent training will not rise to the world-class level unless he or she has inherited a supercharged physiological system for the sport."

The speed of a sprinter is determined in large part by physiology. Muscle proteins, including key energy-producing enzymes, are dictated by genes, as is muscle-fiber composition. Great sprinters, like Maurice Greene and Marion Jones, have a high percentage of fast-twitch muscle fibersfibers that contract quickly but tire quickly too.

A cyclist, in contrast, needs great lung capacity, for superior endurance, and strives for a high "VO2 max," the maximum amount of oxygen the lungs can consume. Lance Armstrong, not surprisingly, has an amazingly high VO2 max.

"Lance Armstrong is not only a supreme trainer and competitor, but he also possesses a Ferrari body," Sparling said. "His aerobic performancean essential key to endurance performanceis one of the highest ever measured in a cyclist."

Great cyclists generally have an extraordinary heart capacity. The average resting heart rate is 66 to 72 beats per minute (bpm). A well-trained endurance athlete has a resting heart rate of 40 bpm. Miguel Indurain, a five-time Tour de France winner and Olympic gold medalist in 1996, recorded a resting heart rate of 28 bpm. In the mountain stages of the Tour de France, Indurain could take his pulse rate up to 190 beats per minute and drop it back to 60 on the descent within half a minute.

To varying degree, these traits are all hereditary. As the renowned Swedish exercise physiologist Per-Olof Åstrand once said, "The most important thing an aspiring athlete can do is to choose the right parents."

Biological Truth

Stephen Gladstone is the director of Intercollegiate Athletics and the head men's crew coach at the University of California, Berkeley. He remembers when Sebastian Bea, a 2000 Olympic silver medalist in rowing, began training in the fall of 1996.

"I saw immediately that he was at a level that could not be arrived at without significant gifts," Gladstone said. "His endurance capabilities were phenomenal. He could go forever."

No wonder: Bea's father was an Olympic basketball player, and his mother was a swimmer.

The performance gap between men and women in sports is also due to genetics. Androgenssex hormones such as testosteronemake males taller, heavier, and more muscular than females.

"When applied to human sports competition, the natural consequence is that men are faster and stronger," Sparling said. "These are simply biological truths associated with being male or female."

But sports scientists emphasize that genetic endowment is simply the stage on which the ultimate performance is created. Physiological gifts are easily squandered without the right training, nutrition, and mental desire.

"There's no one born to be a world-record holder," said Phillip Henson, a professor in the department of kinesiology in Indiana University's School of Health, Physical Education, and Recreation. Henson was the director of competition in charge of track and field at the 1996 Olympics in Atlanta. "People are born with certain abilities, but they have to work with those abilities to become really good."

But genetics, particularly a person's nerve system, also partly determines how well athletes can train and how successfully they can make adjustments and improve their technique.

Henson worked with pole-vaulter Dave Volz, a U.S. record holder and three-time Olympian, who invented a technique in which he used his hand to steady the bar as he went over it.

"No one taught him that," Henson said. "He just had a feel for it that others didn't have."

Physical Limit

So, how much better can elite athletes get?

Some scientists have noted that world records have been broken more seldom in recent Olympics than they were in, say, the 1970s and 1980s. This has led some experts to suggest that human beings are finally approaching the limits of their physical accomplishment.

Gladstone disagrees.

"Every generation feels as if this is itwe can't go any faster, we can't go any further," he said. "Then somebody comes along and breaks the record and the same pundits will be on board to explain why it happened."

But most scientists agree that we have entered an era of smaller improvements, in which the time periods between world records could become longer.

"If you graph the improvement of a sport, the slope of the curve will angle up sharply at first," Henson said. "But as athletes improve, the slope of the curve decreases and begins to level off."

"No one is ever going to run a mile in 60 seconds," he said. "On the other hand, I don't know if we will ever reach the point where we say a record can't be broken."